Sheet feeders for image forming apparatuses typically include a separator to feed one by one sheets of recording media stacked on a sheet tray or sheet cassette, and a feed roller or a pickup roller disposed above the sheets stacked on the sheet tray. The separator can prevent multifeed.
Prior to sheet feeding, the sheets are pressed against the pickup roller, which involves elevation of a bottom plate of the sheet tray. Spring compression methods and motor control methods can be used to elevate the bottom plate.
In spring compression methods, typically a spring is elongated, or a compressed spring is released, by a force to insert the sheet tray into a body of the apparatus, thereby lifting the bottom plate. Although this can be achieved by a simple mechanism at a low cost, a stronger force is required to insert or draw out the sheet tray from the body, thus degrading operability. This also imposes a limitation on the quantity of sheets contained in the sheet tray.
In motor control methods, there is a difficulty in connection between a lift unit to elevate the bottom plate and a motor drive unit for driving the lift unit.
Couplings are widely used for connection structures, and various approaches have been tried to improve coupling connection structures. For example, for elevating the bottom plate of the sheet tray that can be drawn out from the body of the image forming apparatus, JP-H11-079420-A proposes a drive unit that includes a drive shaft projecting from a surface of the sheet tray on the side of the body, a spring pin fitted in the end of the drive shaft in a direction perpendicular to an axial direction, and a coupling driven by a motor. The drive shaft elevates the bottom plate by rotating unidirectionally. An engagement groove in which the spring pin fits is formed in the body, and the coupling is pushed to the sheet tray movably in the axial direction. When the sheet tray is inserted into the image forming apparatus, rotation of the coupling is transmitted to the drive shaft, thereby lifting the bottom plate.
In coupling connection structures, if phases of the elements connected together are shifted from each other, generally a strong force is necessary to insert the sheet tray into the apparatus body. Moreover, those elements cannot be connected together.
Additionally, JP-H06-056283-A proposes the following unit for elevating the bottom plate of the sheet tray. A push-up lever is disposed beneath the bottom plate and fixed to a rotary shaft, and a pressure arm engages and is disengaged from the rotary shaft via the push-up lever and a drive connecting and disconnecting means. This unit further includes a rotating member constructed of a missing-teeth gear and a cam united to the missing-teeth gear, a drive gear that engages the missing-teeth gear to drive the rotating member, a stopper that engages an engagement portion of the cam, a pressure spring stretched between the pressure arm and the rotating member, and a release member to release the stopper from the cam by insertion operation of the sheet tray. When missing-teeth gears are used, a retainer to retain the missing-teeth gear at a predetermined position, a position detector, and the like are necessary.
Additionally, in JP-2003-246468-A, a sector gear is provided to a rotary shaft of a push-up lever to lift the bottom plate. When the sheet tray is mounted in the apparatus and the bottom plate is rotated, the sector gear rotates and is connected to a lift unit, thus pushing the sheet tray in a direction in which the sheet tray is inserted. Then, a member provided to the sheet tray for lifting the bottom plate is fitted in a coupling provided to the body of the apparatus.